I have a Dell Opiplex 755 straight off the shelf that has come preloaded with Windows XP. I am trying to install Windows 2000 over top of it by just booting from the disk. The computer will go into the Win2k and start the initial process of loading the OS. However, as soon as it states "Starting Windows 2000" it gives me the blue screen with STOP: 0x7b...Inaccessible boot device. I don't understand why it won't let me boot from the disk. I have done installs like this plenty of times. Thank you all for your help.